1. Field of the Invention
The present invention relates to a semiconductor device and a fabrication method thereof, and more particularly to a semiconductor device having a triple well structure and a fabrication method thereof.
2. Description of the Prior Art
To meet with the high integration and functional sophistication of semiconductor devices, an integrated circuit generally should be capable of providing special performances. For this purpose, a number of improved semiconductor devices have been developed. In one such improvement, a first P-type well is formed in a P-type substrate, and a second P-type well is formed in a N-type well which is formed in the P-type substrate. Accordingly, a first N-type MOS transistor is formed in the first P-type well, and a second N-type MOS transistor is formed in the second P-type well. The first N-type MOS transistor has properties different from that of the second N-type MOS transistor.
The semiconductor device can be employed in the dynamic RAM. In the dynamic RAM, when a negative voltage having a predetermined value is applied to a semiconductor substrate in a region of a memory cell, parasitic junction capacitance is reduced. Further, quantity of the leakage current in a junction region is reduced. As a result, sensing margin of the semiconductor device and data retention time are increased.
In the conventional semiconductor device as described above, the second P-type well formed in the N-type well is electrically insulated with the first P-type well, and in contrast, to the first P-type well, a negative voltage having a predetermined value is applied to the second P-type well. Accordingly, there is a problem in that threshold voltage in the second P-type well is increased.
Furthermore, since the second P-type well is formed in the N-type well, the characteristic of the second P-type well is deteriorated.